Carbon electrodes from bituminous coal



Patented Apr. 22, 1952 CARBON ELECTRODES FROBI BITUMINOUS COAL Frederick L. Shea, Jr., Chicago, Ill., assignor to Great Lakes Carbon Corporation, Morton Grove, 11]., a corporation of Delaware No Drawing. Application June 15, 1948, Serial No. 33,230

This invention relates to a process for manufacturing carbon electrodes from coal. More particularly it relates to a process for manufacturing carbon electrodes and other shaped carbonaceous bodies from coal as the sole constituent, said coal being autogenously bonded.

The conventional commercial process for manufacturing carbon electrodes comprises mixing a binder such as coal tar pitch or other bituminous hydrocarbonaceous material with calcined volatile free coke generally from petroleum sources. Calcined coal coke has also been used. The gran ular, volatile free coke is generally mixed with coke flour in order to fill the interstices between the larger coke granules, this mixture being compounded with the pitch and the mixture extruded under hi h pressures while hot. The body thus shaped is subjected to baking with a gradual rise in temperature until a temperature within the range of about 900-1800" C. is reached. The material may be baked to a temperature up to about 1000 C. and then may be graphitized in a further step above about 2500 C. Certain bituminous coals when heated to a temperature in the neighborhood of MO-550 C. undergo swelling, passing through a plastic condition at which intumescence occurs. Because of the liberation of excessive amounts of gas, the coke thus produced is generally quite porous and as a result its subsequent use in electrodes is practical only if materials are unavailable which will yield a more dense electrode. Electrodes made in this way have a low apparent density and a high resistivity with the result that power consumption in electrothermal metallurgical processes in which such electrodes are employed is high. As a consequence coal generally cannot compete as a raw material with petroleum coke either on the basis of cost, production or subsequent use.

The formation of carbon electrodes directly from coal is not practical because of the fact that the coal passes through the highly plastic stage and the resulting electrodes are not only deformed but highly porous, blistered 01' cracked.

The present invention provides a method for producing electrodes from coal wherein no added binder is necessary. Electrodes can be made having higher apparent density and greater conductivity than it has been possible to produce by the use of calcined coal coke and a binder.

In one specific embodiment the invention comprises a process for manufacturing carbon electrodes by heating a coking coal at a temperature of about 350-550 C. for about two to about ten hours to a point at which it has become non- 4 Claims. (Cl. 202-26) plastic without the application of mechanical pressure and has about 8 to 20% volatile matter capable of being evolved only upon carbonization, then cooling and crushing the partly ooked coal and then compressing it without added binder at a pressure of at least 2000 pounds per square inch at a temperature of about 350-500 C. to produce a uniform, dense, green electrode, and baking the electrode to a temperature of about 9001100 C.

According to another embodiment the partially devolatilized coal made as described above, is compressed into briquettes or other forms, broken into particles of the desired size for use in electrodes. and calcined at a temperature above 700 C. to a volatile content below about 2% and preferably below 1%. The dense coke thus formed may then be mixed with a binder and made into electrodes of high density and conductivity in the usual manner. The coke is exceptionally hard.

If the coking coals are heated at about 300- 550 C., but at a temperature above that at which destructive distillation begins .to occur and in which the coal becomes plastic, a part of the volatile matter is evolved. By controlling the time of heating and the temperature so that the resultant coke contains about 8 to about 20% of volatile matter the coal becomes non-plastic under ordinary conditions.

By the term non-plastic I mean that the coal will not melt or fuse upon the application of heat alone although heating it too rapidly would cause the development of considerable porosity,

If the pre-coking or partial coking is carried to the point indicated, and then cooled, the material can be broken into fragments of a size use ful in the production of electrodes, for example to a size of about /4 inch in diameter and less depending upon the size of electrode to be made. These particles may then be heated to a temperature of about 350 to about 500 C. and extruded or molded at a pressure of at least 2000 pounds per square inch to produce a uniform dense green electrode. The electrode may then be subjected to baking at a rate more rapid than that which can be employed when using a mixture of calcined coke, coke flour and pitch binder. The temperature of baking is that usually employed in the manufacture of carbon electrodes.

It has been found that the baking operation using small electrodes can be carried out using a temperature rise at a rate from about 10 to 20 C. per minute. In the conventional baking operation for comparable electrodes rates of temperature rise up to about 5 C.-l0 C. per. hour are employed.

This possibility of increased baking rate is highly important in view of the savings that can be affected in the time of manufacture. Larger electrodes, say up to 40 inches in diameter, require upheat rates of a lower order of magnitude than for the smaller sizes, but there are substantial time savings over conventional operation.

Any suitable coking coal can be employed,

these being of the bituminous type normally containing volatile matter within a range of about 20-40%. It is preferred to use coals which have a low ash content either naturally, or which may have been subjected to mechanical and/or chemical treatment to reduce the ash.

The precoking and baking operation is preferably carried out in a non-oxidizing atmosphere.

The precoking operation can be carried out in any suitable apparatus preferably one in which the air can be excluded. This may include vertical retorts, shell coking stills, continuous coking furnaces, and the like.

The following examples are given to illustrate the process but should not be construed as limiting it to the exact conditions or materials therein described.

Example I A Kentucky bituminous coal (Bell) of 37% volatile matter was crushed and heated to a temperature of 430 C. in a stationary retort 'until the volatile content was about 17%. The

heating time was 2 hours. The partially coked coal was removed from the cooked retort, broken into granules having particles passing 50 mesh and then heated in anon-oxidizing atmosphere to 320 C. and extruded at this temperature at a maximum pressure of 10,000 pounds per square inch. The resulting pieces were 1 /2 inches in diameter, then calcined or baked to a temperature of 1250 C. using an upbeat rate of about 10 to 20 C. per minute. The apparent density of the resulting 8-10 mesh coke was 117% of the apparent density of an electrode made from coal coke from the same coal source calcined directly at a heating rate of 5 C. per hour. The coke of this invention contained microscopic voids and was stronger, more dense and had a lower electrical resistivity, i. e., higher electrical conductivity, than the conventional electrode made with the calcined coal coke.

Example II A bituminous coal from West Virginia was precoked to a temperature of about 480 C. over a period of 2 hours. This partially coked material was hard, and did not fuse upon further heating. The partially coked coal was crushed to 8-10 mesh heated in a substantially oxygen free atmosphere to a temperature of 430 C. and was extruded into an electrode at a maximum pressure of 10,000 pounds per square inch. The resulting green electrode was calcined in the manner described in Example I. The calcined briquette showed an apparent density of 123% and a conductance of 140% of that electrode made using volatile free calcined coal coke from the same source in the conventional manner described in Example I. The electrode of this invention was stronger, more dense, and less porous than the conventional electrode. The appearance was considerably better, the cross section was more uniform, the voids contained therein were smaller and in every way the electrode was superior to the conventional one.

Emample IN cining the coal direct. The strength was 50% better, and apparent density about 20% better.

I claim as my invention:

1. A process for manufacturing coal-coke of improved strength, particle density and conductivity, and decreased porosity, suitable for use in electrode manufacture which comprises heating coal consisting essentially of a bituminous coking coal containing about 20-40% volatile matter, and which swells and becomes plastic upon being heated to 400500 'C. to an infusible, normally non-plastic condition at a temperature between about 300 and about 550 C., and in any event to reduce the volatile content by destructive distillation, to between about 8 and about 20%, which material is deformable under high mechanical pressure at a temperature of 300-550 C., cooling and crushing the partially coked coal and compressing it without added binder at a pressure of at least 2,000 pounds per square inch at a temperature between about 350 and about 550 C. to deform and autogenously bond the particles together, crushing the resulting composition to form particles and coking said particles above about 700 C. to a volatile content below about 2%.

2. A process for producing a carbon article which comprises subjecting a coal consisting essentially of bituminous coking coal containing about 20-40% volatile matter, and which swells and becomes plastic upon heating to about 400- 500 C., to destructive distillation at a temperature between about 350-550 C. for a time sufficient to render the resulting material infusible, and non-plastic except upon application of strong mechanical pressure within said temperature range, cooling and crushing the resulting mass, heating the particles thus produced to a temperature between about 350-550" C. and strongly compressing the hot particles at a pressure sufiicient to deform and autogenously bond them together and gradually heating the resulting carbonaceous body to a temperature above 700 C. to reduce its volatile content to below 2 thereby producing a dense, hard, strong carbon article of low electrical resistivity.

3. A process for producing a carbon article which comprises subjecting coal consisting essentially of bituminous coking coal which normally undergoes swelling and passes thru a plastic state at temperatures of 400-550 C., said coal containing 20-40 volatile matter, to destructive distillation at a temperature of about 350-550 C, for a time suflicient to form an infusible. non-swelling mass which is non-plastic, except upon the application of strong mechanical pressure with in said temperature range, and to reduce substantially the volatile content to not less than 8% and not more than 20%, said distillation be ing in a non-oxidizing atmosphere, said time being in the range of 2-10 hours, cooling and then crushing the resulting mass, heating the particles thus produced to a temperature between about 350-550 C. and strongly compressing the hot particles at a pressure sufiicient to deform and autogenously bond them together, and thereafter gradually heating the resulting carbonaceous body to a temperature above 700 C., the rate of heating being not more than about 10 C. per hour, to reduce its volatile content to below about 2%, thereby producing a dense, hard.

strong carbon article of low electrical resistivity.

4. A process for producing a carbon article which comprises subjecting a Kentucky bituminous coal of about 37 volatile matter, and which normally swells and becomes plastic upon heating to about 400-500" 0., to a temperature of about 430 C. for a period of about 2 hours to reduce the volatile content to about 17%, cooling the resulting mass, crushing it to form particles of less than about 50 mesh, heating said particles in a non-oxidizing atmosphere at a temperature of about 320 C., compressing the thus heated particles at a pressure sufiicient to deform and autogenously bond them together, thereafter gradually heating the resulting carbonaceous body to a temperature of about 700 C. to reduce the volatile content to below about 2%, thereby producing a dense, hard, strong carbon article of low electrical resistivity.

FREDERICK L. SHEA, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,538,796 Gordon May 19, 1925 1,675,674 Szarvasy July 3, 1928 1,909,421 Parr et al. May 16, 1933 2,025,776 Roberts Dec. 31,1935 2,365,055 Cole Dec. 12, 1944 2,376,760 Elsey May 22, 1945 2,461,365 Bennett et al. Feb. 9, 1949 FOREIGN PATENTS Number Country Date 367,193 France Feb. 13, 1908 319,895 Great Britain Oct. 3, 1929 356,236 Great Britain Sept. 7, 1931 357,330 Great Britain Sept. 24, 1931 358,181 Great Britain Oct. 8, 1931 445,208 Great Britain Apr. 6, 1936 

1. A PROCESS FOR MANUFACTUREING COAL-COKE OF IMPROVED STRENGTH, PARTICLE DENSITY AND CONDUCTIVITY, AND DECREASED POROSITY, SUITABLE FOR USE IN ELECTRODE MANUFACTURE WHICH COMPRISES HEATING COAL CONSISTING ESSENTIALLY OF A BITUMINOUS COKING COAL CONTAINING ABOUT 20-40% VOLATILE MATTER, AND WHICH SWELLS AND BECOMES PLASTIC UPON BEING HEATED TO 400-500* C. TO AN INFUSIBLE, NORMALLY NON-PLASTIC CONDITION AT A TEMPERATURE BETWEEN ABOUT 300* AND ABOUT 550* C., AND IN ANY EVENT TO REDUCE THE VOLATILE CONTENT BY DESTRUCTIVE DISTILLATION, TO BETWEEN ABOUT 8 AND ABOUT 20%, WHICH MATERIAL IS DEFORMABLE UNDER HIGH MECHANICAL PRESSURE AT A TEMPERATURE OF 300-550* C., COOLING AND CRUSHING THE PARTIALLY COKED COAL AND COMPRESSING IT WITHOUT ADDED BINDER AT A PRESSURE OF AT LEAST 2,000 POUNDS PER SQUARE INCH AT A TEMPERATURE BETWEEN ABOUT 350* AND ABOUT 550* C. TO DEFORM AND AUTOGENOUSLY BOND THE PARTICLES TOGETHER, CRUSHING THE RESULTING COMPOSITION TO FORM PARTICLES AND COKING SAID PARTICLES ABOVE ABOUT 700* C. TO A VOLATILE CONTENT BELOW ABOUT 2%. 